1. Field of the Invention
This invention relates generally to wellbore construction and more particularly to the construction of one or more wellbores from a large diameter wellbore that can act as a subsurface platform for drilling multiple wellbores and provide space for housing a variety of equipment that can be utilized to perform a host of functions or operations during the drilling, completion and production phases of such wellbores.
2. Background of the Art
To obtain hydrocarbons such as oil and gas, wellbores or boreholes are drilled from one or more surface locations into hydrocarbon-bearing subterranean geological strata or formations (also referred to in the industry as reservoirs). A large proportion of the current drilling activity involves drilling deviated and/or substantially horizontal wellbores extending through such reservoirs. To develop an oil and gas field, especially offshore, multiple wellbores are drilled from an offshore rig or platform stationed at a fixed location. A template is placed on the sea bed, which template defines the location and size of each of the multiple wellbores to be drilled. The various wellbores are then drilled from the template along their respective pre-determined wellpaths (or drilling course) to their respective depths. Frequently, ten to twenty offshore wellbores are drilled from an offshore rig stationed at a single location. In some regions, such as the North Sea, as many as sixty separate wellbores have been drilled from an offshore platform stationed at a single location. The initial drilling direction of several thousand feet of each such wellbore is generally vertical and typically lies in a non-producing (non-hydrocarbon bearing) formation.
Each wellbore is then completed to produce hydrocarbons from its associated subsurface formations. Completion of a wellbore typically includes placing casings through the entire length of the wellbore, perforating production zones and installing safety devices, flow control devices, zone isolation devices and other devices within the wellbore. Additionally, each wellbore has associated welihead equipment, generally referred to as a "tree" and includes a blow-out-preventor, a riser or stuffing box and other safety and fluid flow control devices.
The above-described wellbore construction requires drilling each such wellbore from the surface to its respective depth and then completing each such wellbore to its entire depth. Typically, the first several thousand feet of each such wellbore lie in non-hydrocarbon-producing formations. As an example, assume that there are ten wellbores drilled from an offshore platform, each having a nine-inch internal diameter. Further assume that there is no production zone for the initial five thousand feet for any of these wellbores. In this example, there would be a total fifty thousand feet (five thousand for each of the ten (10) wellbores) of non-producing wellbore that must be drilled and completed, serving little useful purpose. It is, therefore, desirable to drill as few upper portions as necessary from a single location or site, especially as the cost of drilling and completing offshore wellbores can exceed several tens of thousand dollars per each thousand feet of wellbore.
The production wellbores are relatively small in diameter and typically contain flow tubings and a variety of devices, leaving little or no space for installing other equipment, such as fluid processing equipment to separate oil, gas and water, compressors for transporting gas and fluids uphole, chemical injection equipment for treating fluids downhole, etc. Therefore, the above-described equipment and wellhead equipment that includes safety devices such as blow-out-preventor stacks, are usually installed on the earth's surface. Oil, gas and water separators are typically installed adjacent to the wellheads. During the production phase, fluids from each of the production wellbores are passed to the separator, which separates the constituents, which are then transported to their desired destinations.
The above-described equipment, whether installed on the earth's surface or on the sea bed, poses environmental risks and is susceptible to theft and damage. The cost to the wellbore operators can be significant if the equipment is damaged or if a blow-out occurs. It is therefore desirable to install much of the above-described equipment and power sources below the earth's surface. The wellbore construction methods of the present invention allow installing a variety of equipment below the earth's surface.
An important aspect of drilling a deviated or horizontal wellbore is to drill it along a predetermined wellpath or drilling course. During drilling of the wellbore, it is important to accurately determine the true location of the drill bit relative to a reference point so as to continuously orient or maintain the drill bit along the desired wellpath. This would require substantially continuously transmitting data relating to the drill bit location to the surface. However, the current drill strings usually include a large number of sensors to provide information about the drill bit location, formation parameters, borehole parameters and the tool condition and a relatively low data transmission telemetry, such as the mud-pulse telemetry. In such systems, the drill bit location data is transmitted to the surface periodically. The drill bit sometimes significantly veers off course, requiring larger adjustments to maintain the drilling course, which can be very time consuming. The present invention provides a method for continuously determining the drill bit location which allows the driller to drill the wellbore along the predetermined wellpath.
During the completion of a wellbore, a number of devices are utilized in the wellbore to perform specific functions or operations. Such devices may include, packers, sliding sleeves, perforating guns, fluid flow control devices, and a number of sensors. To efficiently produce hydrocarbons from wellbores drilled from a single location or from multi-lateral wellbores, various remotely-actuated devices can be installed to control fluid flow from various subterranean zones. Some operators are now permanently installing a variety of devices and sensors in the wellbores. Some of these devices, such as sleeves, can be remotely controlled to control the fluid flow from the producing zones into the wellbore. The sensors are used to periodically provide information about formation parameters, condition of the wellbore, fluid properties, etc. One problem with this approach is that the distance between the transmitter and certain sensors in the wellbore can be great, necessitating the installation of relatively large power sources in the wellbore for providing power to such devices and sensors. Additionally, the quality of the data received from the sensors can suffer because of the large distance between such devices and the surface. It is therefore desirable to locate the power sources and communication and data processing devices closer to the downhole devices and sensors.
The vast majority of the oilfields contain a plurality of spaced wellbores. Each such wellbore requires services during its production life. Such services are provided from the wellhead equipment installed at the uphole end of each such wellbore. In one embodiment of the present invention, one or more access wellbores are drilled at strategic locations in the field, wherein each such access wellbore is utilized to service a plurality of wellbores.
The present invention addresses the above-described problems with the prior art methods for constructing multiple wellbores from a single location and multi-lateral wellbores. In one method, the present invention provides for constructing a single relatively large diameter deep wellbore (access or main wellbore) and then forming a plurality of relatively small diameter wellbores from near the bottom of the main wellbore. The single main wellbore can significantly reduce the overall drilling length and provides space for installing certain types of equipment within the main wellbore. One or more of the smaller wellbores may be formed in non-producing formations in the vicinity of a group of production wellbores for storing power supplies, transmitters and receivers, data processing equipment to aid the drilling of other wellbores and to aid production of hydrocarbons from the production wellbores.